A number of clinically important drugs such as the epipodophyllotoxins
etoposide (VP-16) and
teniposide (VM-26), the
anthracycline daunorubicin and
doxorubicin (
Adriamycin), and the
aminoacridine amsacrine exert their cytotoxic action by stabilizing the cleavable complex formed between
DNA and the nuclear
enzyme topoisomerase II. We have previously demonstrated in several in vitro assays that the
anthracycline aclarubicin (
aclacinomycin A) inhibits cleavable-complex formation and thus antagonizes the action of drugs such as
VP-16 and
daunorubicin. The present study was performed to validate these in vitro data in an in vivo model. At nontoxic doses of 6 and 9 mg/kg,
aclarubicin yielded a marked increase in the survival of non-
tumor-bearing mice given high doses of
VP-16 (80-90 mg/kg) in six separate experiments. In
therapy experiments on mice inoculated with
Ehrlich ascites tumor cells,
aclarubicin given at 6 mg/kg roughly halved the increase in median life span induced by
VP-16 at doses ranging from 22 to 33 mg/kg. An attempt to determine a more favorable combination of
VP-16 and
aclarubicin by increasing
VP-16 doses failed, as the two drugs were always less effective than
VP-16 alone. The way in which VP-16-induced
DNA strand breaks lead to cell death remains unknown. However,
VP-16 has been reported to cause apoptosis (programmed cell death) in several cell lines. To ascertain whether the protection given by
aclarubicin could have a disruptive effect on the apoptotic process, we used the small intestine as an in vivo model. Whereas VP-16-induced apoptosis in crypt stem cells was detectable at a dose as low as 1.25 mg/kg,
aclarubicin given at up to 20 mg/kg did not cause apoptosis. Indeed,
aclarubicin caused a statistically significant reduction in the number of cells rendered apoptotic by
VP-16. The present study thus confirms the previous in vitro experiments and indicates the value of including an in vivo model in a preclinical evaluation of
drug combinations.